Single crystals of cerium doped lutetium yttrium orthosilicate scintillator (Ce:(Lu(1-X)YX)2SiO5; Ce:LYSO) were grown through Czochralski method to investigate the affection of different growth conditions on the crystal growth. The relationship between thermal field and crystal growth was investigated. Different growth parameters and insulating system were adopted and compared to decrease the temperature gradient. It is found that the new insulating bricks can decrease effectively the temperature gradient and solve the crack problem of grown crystal. The relationship between materials ratio and the impurities in crystals. It is found that the optimized material ratio is Lu2O3:Y2O3:CeO2:SiO2=1.8563:0.1397:0.008:1.99. Under this ratio, we can get the crystals with best macroscopic optical quality. The scintillation properties of grown Ce:LYSO single crystal were measured. Through comparing the data with those in published literatures, it is found that the grown Ce:LYSO crystals present outstanding optical transmission, reaching the theoretical value about 84% in the visible light region. The transmission, UV-excitation and emission spectra are almost identical with the reported ones. The luminescence efficiency (LE) of random chosen 43 Ce:LYSO¡@samples were measured on the £^-ray multi-channel energy spectrum. It is found that the Ce:LYSO samples present stable luminescence efficiency with minimum channel number 3636.37, maximum channel number 4293.78, maximum deviation of 9.91% and standard deviation of 152.24 (3.77%). Annealing treatments were carried out in the air atmosphere on the small pieces of Ce:LYSO single crystals to eliminate the oxygen vacancies in the sample. It is found that the optimized annealing condition to improve the luminescence efficiency of Ce:LYSO is 1400 ¢J, 80 h under air atmosphere.

¡@¡@This paper mainly focuses on the discussion about how to grow a large-sized highly-doped laser crystal- Neodymium yttrium aluminum garnet( {NdxY1-x}3Al5O3; Nd:YAG). The higher concentration of Nd:YAG crystal can allow the better efficiency of the laser. Since the ion size doped with Nd3+ ( R= 1.12A ) is larger than the ion size of Y3+ (R= 1.01A ), in the lattice of YAG, it¡¦s not easy to mix the smaller Y3+ site with the larger diameter of the Nd3 + ion. Therefore, the higher concentration of the laser crystal we want to grow, the more difficult work it is. ¡@¡@This experiment works under the use of the Czochralski technique to grow the laser crystal: Nd:YAG, and explore the impacts of different crystal pulling growth conditions on the growth of Nd:YAG crystal. Through adjusting the parameters of crystal growth, the crystal growth environments and the thermal fields, I discuss how to solve the problems of the scattering, cracking and spiral growing during the crystal growth process in order to improve the quality of the crystals. ¡@¡@Finally, comparing the results of a variety of spectral analysis (X-ray diffraction, UV / Vis Spectroscopy, Raman Spectroscopy, PL Spectroscopy) on the slice samples of Nd:YAG crystal which grew by our laboratory with the literature results, we can find that the X-ray diffraction pattern, the absorption spectroscopy, the Raman spectroscopy and the Photoluminescence spectroscopy of the Nd:YAG crystals grew in this experiment are consistent with the literature.

No description available.

The field of low-dimensional quantum magnets has received lot of attention owing to the possibility of studying phenomena associated with the quantum nature of matter. Many materials that realize low-dimensional spin arrangements in their structure have been synthesized in the past twenty years due to the emergence and development of crystal growth techniques. These materials have been studied using various experiments in order to explore the wide range of interesting properties predicted theoretically for low-dimensional systems. In this pursuit, novel properties have been observed and many open questions have been raised. One such property that is typically observed in many low-dimensional quantum magnets is heat transport via magnetic excitations. Large magnitudes of magnetic heat conductivity has been found experimentally in materials belonging to this class in addition to the conventionally known phononic heat conduction, and interesting theoretical predictions like the divergence of heat conductivity in certain spin models exist, that have stimulated research in this field. This experimental work mainly deals with the crystal growth and heat transport properties of low-dimensional quantum magnets that include one-dimensional (1D) spin chain systems Sr$_2$CuO$_3$ and SrCuO$_2$, two-dimensional (2D) Heisenberg antiferromagnet La$_2$CuO$_4$, and a five-leg spin ladder La$_8$Cu$_7$O$_{19}$, with a view to understand propagating low-energy magnetic excitations and their interaction amongst themselves, other quasiparticles and impurities present in the systems. These interactions result in scattering processes that govern the magnitude and temperature dependence of heat conductivity. In spite of considerable theoretical and experimental work in the field of heat transport, a complete understanding of the scattering mechanisms is lacking. The work tries to add to the experimental knowledge about magnetic heat transport in such systems and presents cases which motivate the need for theoretical understanding of aspects of heat transport. The focus of this work was twofold. One part focusses on the single crystal growth using the travelling-solvent floating zone (TFSZ) method of materials which realize low-dimensional spin systems in their structure. The TFSZ method is indispensable for growing large single crystals of extraordinary purity, which can be used for investigations using neutrons and other techniques like heat conductivity measurements that probe anisotropic properties. The other part deals with the experimental results on heat transport and other thermodynamic properties of these materials. In order to study the behaviour of the magnetic heat conductivity at high temperatures, and the effect of small amount of magnetic and non-magnetic impurities on the heat transport of 2D Heisenberg antiferromagnet La$_2$CuO$_4$, single crystals of pure La$_2$CuO$_4$, and Ni- and Zn-doped versions, La$_2$Cu$_x$Ni$_{1-x}$O$_4$ and La$_2$Cu$_x$Zn$_{1-x}$O$_4$ for $x$ = 0.001 and 0.003, were grown using the TFSZ method. Heat transport in the pure compound was experimentally investigated for the first time up to very high temperatures of 813 K using two methods, namely the steady state method for low temperatures and the dynamic flash method for measuring high temperature conductivity. Analysis of the magnon mean-free path using empirical models based on semi-classical theories, and qualitative comparison to theoretical calculations seems to suggest that scattering between magnons might play an important role in addition to scattering of magnons with phonons and defects, and that the spin-spin correlation length could be crucial in limiting the mean free path of magnons at high temperatures. These experimental results and indications of probable scattering mechanisms based on non-rigorous analyses and comparisons, strongly motivate the need for theoretical studies. Heat conductivity measurements on the Ni- and Zn- doped versions of La$_2$CuO$_4$ are still incomplete and inconclusive, and hence have not been reported in this work. Heat transport experiments on Ni- and Ca-doped Sr$_2$CuO$_3$ were performed, with a motivation to investigate the role of disorder induced by impurities lying within the spin chains (Ni) and those lying outside the spin chains (Ca), on the heat transport in this system. In both the cases, the magnetic heat transport is observed to be strongly suppressed upon doping. Empirical analysis of the data seems to suggest that in the temperature regime of 100-300 K, the temperature dependence of the mean-free path of magnetic excitations for the Ni- and Ca-doped samples can be described by scattering with defects (Ni and Ca impurities) and phonons alone. However, surprisingly, a strong increase of phononic conductivity is observed perpendicular and parallel to the spin chains of the Ni-doped compounds compared to the pure compounds, whose explanation seems to lie in the existence of an additional dissipative scattering mechanism present in the pure compounds that is lifted upon doping, possibly due to the presence of a spin gap in the doped compounds. The effect of Ni on the Sr$_2$CuO$_3$ and SrCuO$_2$ was also investigated by studying the low energy regime of the spin excitation spectrum using other microscopic probes like nuclear magnetic resonance (NMR) and inelastic neutron scattering (INS). Large single crystals of SrCu$_x$Ni$_{1-x}$O$_2$, with $x$ = 0.01 were grown and used in these experiments that observed the presence of a spin gap in the Ni-doped sample. Further theoretical investigations are however required to understand the possible role of the spin gap in influencing the spin-phonon scattering mechanism, and its relevance to the observed enhancement in phononic conduction. Although we observe that in the case of both 1D and 2D systems, a semi-classical kinetic model for heat transport along with empirical models of scattering processes describe the temperature dependence of the measured heat conductivity surprisingly well in the temperature regime up to 300 K and 800 K respectively, interpretations based on these analyses must be treated as only preliminary, and as a step towards understanding microscopically the scattering mechanisms involved in low-dimensional systems such as the ones discussed in this work. In the direction of exploratory research towards synthesis of novel low-dimensional materials, two cuprate compounds were synthesized in the form of single crystals using the floating zone method for the first time, namely, a five leg $S=tfrac{1}{2}$ antiferromagnetic spin ladder compound La$_8$Cu$_7$O$_{19}$ and an insulating delafossite LaCuO$_{2}$. A bulk 3D antiferromagnetic ordering is observed in La$_8$Cu$_7$O$_{19}$. Heat conductivity of La$_8$Cu$_7$O$_{19}$ is observed to be purely phononic and no contribution from magnetic excitations seem to exist, although the measurements indicates that there is a large anisotropy in heat transport. However, detailed diffraction experiments using x-rays and neutrons indicate that both the crystal and magnetic structures are complicated, and that the details of the structure prevent La$_8$Cu$_7$O$_{19}$ from being a perfect realization of a five-leg spin ladder.

In dieser Arbeit wurde die Ausgangsverbindung der eisenbasierten Supraleiter, LaFeAsO, durch die Synthese und Charakterisierung von poly- und einkristallinen Proben untersucht. Supraleitung kann in den eisenbasierten Supraleitern durch Elektronen- oder Lochdotierung hervorgerufen werden. Die Substitution von Eisen durch Mangan, formal eine Lochdotierung, hat hingegen einen destruktiven Effekt auf die Supraleitung. Dieser ist bei optimal fluordotiertem LaFeAs(O,F) um Größenordnungen stärker ausgeprägt als bei Nd- oder Sm-FeAs(O,F). Indem Lanthan partiell durch das kleinere Yttrium substituiert wurde, konnte gezeigt werden, dass diese unterschiedlich starke Mangantoleranz durch die Unterschiede im Seltenerdmetall-Ionenradius bedingt ist. Weiterhin finden sich Anzeichen, dass die Unterdrückung der Supraleitung durch Mangan mit Elektronenlokalisierung korreliert ist. Das Fehlen von großen dreidimensionalen Einkristallen der SEFeAsO-Verbindungsklasse stellt ein großes Hindernis in der Erforschung der elektronischen Eigenschaften der eisenbasierten Supraleiter dar. Im Rahmen dieser Arbeit konnte gezeigt werden, dass das Verfahren der Festkörper-Einkristallzüchtung ein geeignetes Mittel darstellt, um große, facettierte SEFeAsO-Einkristalle mit ausgeprägtem Wachstum in c-Richtung zu erhalten. Mit diesem neu entwickelten Einkristallzüchtungsverfahren konnte ein aktualisiertes Phasendiagramm von La(Fe,Co)AsO erstellt werden. Die Substitution von Eisen durch Cobalt entspricht einer Elektronendotierung und führt zu Supraleitung mit einer maximalen Sprungtemperatur von 12 K. Die Ausgangsverbindung LaFeAsO zeigt bei etwa 156 K einen strukturellen Phasenübergang von einer tetragonalen zu einer orthorhombischen Struktur, weiterhin tritt unterhalb von etwa 138 K eine Spindichtewelle auf. In Einklang mit dem bekannten Phasendiagramm werden mit Cobaltdotierung die beiden Übergänge unterdrückt, mit höheren Cobaltkonzentrationen kommt es zu Supraleitung. Anders als beim bekannten Phasendiagramm kann eine deutliche Aufspaltung zwischen magnetischem und strukturellen Übergang bei kleinen Cobaltkonzentrationen beobachtet werden. Außerdem findet sich eine Region der Koexistenz zwischen Supraleitung und Spindichtewelle. Bisher konnte ein solcher Zustand im SE(Fe,Co)AsO-System nicht beobachtet werden.

This experimental work focusses on the magnetic thermal conductivity, κ_mag, of the one-dimensional two-leg spin ladder system Sr_14Cu_24O_41 and the spin chain system SrCuO_2. These two S = 1/2 antiferromagnetic Heisenberg compounds possess enormous magnetic contributions to the heat transport which in some cases exceed the phonon contributions by more than one order of magnitude. Despite of intense ongoing experimental and theoretical investigations, the underlying mechanism of the magnetic heat transport remains unclear. The study of κ_mag aims a better understanding of the basic physics which determine mobility, scattering and dissipation of the dispersing magnetic excitations. The most important tool used in this study is to selectively influence the structure and the electronic and magnetic properties of the compounds through doping. For this purpose single crystalline samples were produced using the Traveling Solvent Floating Zone technique, a crucible-free technology, which allows the growth of centimeter sized single crystals of high quality. In particular, the successful growth of large quantities of the hole-free ladders La_4Sr_10Cu_24O_41 allowed the realization of inelastic neutron scattering and, for the first time, the acquisition of the complete magnetic excitation spectrum of the spin ladder, composed not only by the triplon band, but also by the two-triplon continuum, permitting an accurate determination of the coupling constants in this system. The importance of the cyclic-exchange, previously unclear, was asserted. In order to study the scattering mechanisms of the magnetic excitations (triplons) off static defects in the two-leg ladder Sr_14Cu_24O_41, this compound was doped with tiny amounts of Zn. Occupying the Cu site in the ladders, the Zn plays the role of a non-magnetic defect, imposing an upper limit to the mean free path of the triplons. The thermal conductivity of Sr_14Cu_(14−z)Zn_zO_41, with z = 0, 0.125, 0.25, 0.5 and 0.75, shows a strong decrease of both the phononic and magnetic contributions with increasing z value. In particular, the decrease of the magnetic part indicates an increased scattering of the triplons off Zn defects. The analysis of κ_mag, using a kinetic model, allows the extraction of the triplon mean free path l_mag. This quantity was successfully correlated to the mean Zn-Zn distance along the ladders, confirming the validity of the employed kinetic model and corroborating results of previous works. In SrCuO_2, the magnetic contribution to the thermal conductivity appears as a hump-like anomaly on the high-T back of the low-T phonon peak. In order to better separate the magnetic contribution from the phononic background, small amounts of Sr were substituted by the smaller and lighter Ca, leading, on the one hand, to an increased scattering of the phonons and consequently to a suppression of the phononic thermal conductivity. On the other hand, since Ca is isovalent to Sr, no significant changes of the magnetic properties of the system are expected: a magnetic peak belonging to κ_mag should appear. Measurements of the thermal conductivity of Sr_(1−x)Ca_xCuO_2 for x = 0, 0.0125, 0.025, 0.05 and 0.1 show indeed a systematic decrease of the phonon thermal conductivity with increasing x. However, against initial expectations, no magnetic peak appears. Instead, the magnetic thermal conductivity decreases at intermediate and low temperatures with increasing doping level, indicating a strong influence of the Ca dopant on the magnetic system. Surprisingly, no changes of κ_mag occur at higher temperatures, where κ_mag remains constant for all doping levels. To explain this intriguing temperature and doping dependence of κ_mag, three scenarios are proposed. One of the scenarios is based on the phenomenon of mutual spinon and phonon heat transport, the so called spin-phonon-drag mechanism. Another scenario assumes an effective scattering of spinons off Ca defects. In a third scenario, the appearance of a gap in the doped compounds is considered. The obvious effect of the Ca dopant on the magnetic thermal conductivity motivated a more detailed investigation of the doping dependence of electronic and magnetic properties in Sr_(1−x)Ca_xCuO_2. NMR data reveal the presence of a magnetic gap for the x = 0.1 compound. The doping dependent evolution of the specific heat at low-T is consistent with this result. Furthermore, susceptibility data may be explained within a segmentation of the spin chains, which in turn can be also related to the opening of a gap. These results strongly support that the reduction of κ_mag in the Ca doped compounds is related to a smaller number of magnetic excitations participating in the heat transport due to the presence of the gap. A possible reduction of the chain length, as suggested by the susceptibility data, is also consistent with the scenario of a reduced κ_mag due to an increased scattering of magnetic excitations. In spite of these partially consistent results, there are still no clear-cut explanations for the evolution of κ_mag upon doping. In particular, it cannot be completely ruled out that a fraction of the Ca dopant goes into the chains, a point which has to be urgently clarified in order to allow a correct interpretation of the data.

Die Dissertation beschreibt Untersuchungen der intermetallischen Verbindungen TbxY1-xNi2B2C bzw. TbxEr1-xNi2B2C, Tb2PdSi3 sowie Dy2PdSi3, die sich auszeichnen durch physikalische Erscheinungen, die durch die Wechselwirkung der magnetischen Momente der Seltenerd (SE)-Ionen mit den Leitungselektronen hervorgerufen werden. Sie umfasst Beiträge zur Konstitution der Legierungssysteme, methodische Untersuchungen zur Einkristallzüchtung, zur Charakterisierung der Homogenität und Perfektion der Kristalle und zu physikalischen Eigenschaften (Supraleitung, Magnetismus). Die für das Erstarrungs- und Schmelzverhalten der intermetallischen Phasen relevanten quasi-binären Schnitte der Tb-(Y, Er)-Ni-B-C bzw. Tb(Dy)-Pd-Si-Phasendiagramme, die bisher noch nicht bekannt waren, konnten bestimmt werden. Damit wurden die Prozessparameter der Einkristallzüchtung durch tiegelfreies Zonenschmelzen optimiert. Die unterschiedlichen Erstarrungstypen, peritektische Erstarrung (SENi2B2C) bzw. kongruentes Erstarren (SE2PdSi3), wurden durch unterschiedliche Ziehgeschwindigkeiten bei der Einkristallzüchtung berücksichtigt. An den massiven Einkristallen wurden Homogenitätsuntersuchungen hinsichtlich Elementkonzentration und physikalischer Eigenschaften (Tc, Tn, RRR) als Funktion der Längskoordinate durchgeführt, die geringe Eigenschaftsunterschiede der einkristallinen Proben nachweisen. Aus der Zusammensetzungsänderung über die Kristalllänge konnte in Verbindung mit in-situ Messungen der Zonentemperatur ein einfaches Prozessmodell des Zonenschmelzens der untersuchten Verbindungen entwickelt werden. Die Korrelation von magnetischen und supraleitenden Eigenschaften der Borokarbidmischreihen werden primär vom Verhältnis der Seltenen Erden bestimmt aber auch durch die Konzentration der weiteren Elemente. An Einkristallen konnte eine Anisotropie des oberen kritischen Feldes der Supraleitung von TbxY1-xNi2B2C gezeigt werden, die durch die magnetischen Tb-Ionen bestimmt wird. Die Untersuchungen der Tb2PdSi3- und Dy2PdSi3-Einkristalle ergaben eine Anisotropie der magnetoelektrischen Eigenschaften. / In this thesis investigations of the intermetallic compounds TbxY1-xNi2B2C, TbxEr1-xNi2B2C, Tb2PdSi3 and Dy2PdSi3 are presented. These compounds exhibit interesting physical phenomena caused by the interaction of the rare earth (RE) magnetic moments on the conduction electrons. Moreover, contributions on the constitution of the alloy systems, basic investigations of crystal growth process, homogeneity, microstructure and physical properties (superconductivity, magnetism) of the crystals are given. The quasi-binary sections of the Tb-(Y, Er)-Ni-B-C and Tb(Dy)-Pd-Si-phase diagrams which are relevant for the crystallisation of the different intermetallic phases have been determined for the first time. They were utilised for optimisation of the process parameters of single crystal growth by floating-zone melting. Because of the different solidification modes of RENi2B2C (peritectic solidification) and RE2PdSi3 (congruent solidification) different growth velocities have been employed in crystal growth. The composition and the physical properties (Tc, Tn, RRR) have been investigated as function of the crystal axis co-ordinate. As these properties show only a slight shift over the crystal length samples are representative for the whole crystal. From the slight composition shift over the crystal length a process model of the floating zone growth has been developed utilising the in-situ measurements of the zone-temperature. The correlation of magnetic and superconducting properties of the borocarbide solid solution compounds are mainly governed by the RE-fraction but they are also influenced by the concentration of the other elements. For TbxY1-xNi2B2C single crystals an anisotropic upper critical field of superconductivity has been detected which is induced by the magnetic Tb-ions. The investigations of Tb2PdSi3 and Dy2PdSi3 revealed an anisotropy of the magnetoresistive properties.

In der vorliegenden Arbeit werden wissenschaftlich-technologische Untersuchungen zur wandabgelösten VGF-Züchtung von Germanium präsentiert. Dazu wurden zwei Varianten mit Hinblick auf die Etablierung und Stabilisierung bestimmter Druckverhältnisse angewendet: (i) Beim passiven dVGF-Verfahren erfolgt die Erzeugung der Druckdifferenz über die thermische Beeinflussung des Inertgases Ar, und (ii) beim erstmals gezeigten aktiven dVGF-Verfahren über eine temperaturkontrollierte, separate Zn-Dampfdruckquelle. Ge-Einkristalle mit einem Durchmesser bis zu 3 Zoll wurden nahezu vollständig ohne Tiegelkontakt gezüchtet. Der Effekt der Wandablösung wird anhand der mikroskopischen Charakterisierung der Kristalloberfläche, der Durchbiegung der Phasengrenze sowie der etch pit density (EPD), ein Maß für die Versetzungsdichte gezeigt. Im Vergleich zu konventionell gezüchteten VGF-Kristallen zeigen die detached gezüchteten Kristalle eine hohe strukturelle Qualität. Dies wird auf reduzierte thermische und thermo-mechanische Spannungen bei der wandabgelösten Züchtung zurückgeführt.

Die Dissertation beschreibt Untersuchungen der intermetallischen Verbindungen TbxY1-xNi2B2C bzw. TbxEr1-xNi2B2C, Tb2PdSi3 sowie Dy2PdSi3, die sich auszeichnen durch physikalische Erscheinungen, die durch die Wechselwirkung der magnetischen Momente der Seltenerd (SE)-Ionen mit den Leitungselektronen hervorgerufen werden. Sie umfasst Beiträge zur Konstitution der Legierungssysteme, methodische Untersuchungen zur Einkristallzüchtung, zur Charakterisierung der Homogenität und Perfektion der Kristalle und zu physikalischen Eigenschaften (Supraleitung, Magnetismus). Die für das Erstarrungs- und Schmelzverhalten der intermetallischen Phasen relevanten quasi-binären Schnitte der Tb-(Y, Er)-Ni-B-C bzw. Tb(Dy)-Pd-Si-Phasendiagramme, die bisher noch nicht bekannt waren, konnten bestimmt werden. Damit wurden die Prozessparameter der Einkristallzüchtung durch tiegelfreies Zonenschmelzen optimiert. Die unterschiedlichen Erstarrungstypen, peritektische Erstarrung (SENi2B2C) bzw. kongruentes Erstarren (SE2PdSi3), wurden durch unterschiedliche Ziehgeschwindigkeiten bei der Einkristallzüchtung berücksichtigt. An den massiven Einkristallen wurden Homogenitätsuntersuchungen hinsichtlich Elementkonzentration und physikalischer Eigenschaften (Tc, Tn, RRR) als Funktion der Längskoordinate durchgeführt, die geringe Eigenschaftsunterschiede der einkristallinen Proben nachweisen. Aus der Zusammensetzungsänderung über die Kristalllänge konnte in Verbindung mit in-situ Messungen der Zonentemperatur ein einfaches Prozessmodell des Zonenschmelzens der untersuchten Verbindungen entwickelt werden. Die Korrelation von magnetischen und supraleitenden Eigenschaften der Borokarbidmischreihen werden primär vom Verhältnis der Seltenen Erden bestimmt aber auch durch die Konzentration der weiteren Elemente. An Einkristallen konnte eine Anisotropie des oberen kritischen Feldes der Supraleitung von TbxY1-xNi2B2C gezeigt werden, die durch die magnetischen Tb-Ionen bestimmt wird. Die Untersuchungen der Tb2PdSi3- und Dy2PdSi3-Einkristalle ergaben eine Anisotropie der magnetoelektrischen Eigenschaften. / In this thesis investigations of the intermetallic compounds TbxY1-xNi2B2C, TbxEr1-xNi2B2C, Tb2PdSi3 and Dy2PdSi3 are presented. These compounds exhibit interesting physical phenomena caused by the interaction of the rare earth (RE) magnetic moments on the conduction electrons. Moreover, contributions on the constitution of the alloy systems, basic investigations of crystal growth process, homogeneity, microstructure and physical properties (superconductivity, magnetism) of the crystals are given. The quasi-binary sections of the Tb-(Y, Er)-Ni-B-C and Tb(Dy)-Pd-Si-phase diagrams which are relevant for the crystallisation of the different intermetallic phases have been determined for the first time. They were utilised for optimisation of the process parameters of single crystal growth by floating-zone melting. Because of the different solidification modes of RENi2B2C (peritectic solidification) and RE2PdSi3 (congruent solidification) different growth velocities have been employed in crystal growth. The composition and the physical properties (Tc, Tn, RRR) have been investigated as function of the crystal axis co-ordinate. As these properties show only a slight shift over the crystal length samples are representative for the whole crystal. From the slight composition shift over the crystal length a process model of the floating zone growth has been developed utilising the in-situ measurements of the zone-temperature. The correlation of magnetic and superconducting properties of the borocarbide solid solution compounds are mainly governed by the RE-fraction but they are also influenced by the concentration of the other elements. For TbxY1-xNi2B2C single crystals an anisotropic upper critical field of superconductivity has been detected which is induced by the magnetic Tb-ions. The investigations of Tb2PdSi3 and Dy2PdSi3 revealed an anisotropy of the magnetoresistive properties.

The ground-state properties of f-electron compounds are characterized by strong interactions which lead to a plethora of exotic states. In this thesis we focused on compounds crystallizing in the tetragonal HonCoGa3n+2-type structure. Several new compounds have been synthesized from In-flux; particularly enigmatic are Ce3TIn11 (T = Pd, Pt). These compounds possess two inequivalent Ce-sites and reveal coexistence of antiferromagnetic order and heavy fermion superconductivity at ambient pressure. This coexistence can be explained in the framework of two-Kondo ion lattice model, assuming one Ce-site being completely Kondo screened with the 4f-electron behaving itinerant and participating in superconductivity while the second Ce-site retains its local character giving rise to magnetic order via the RKKY interaction. The influence of crystal electric field on the magnetic ground state has been investigated on the Kondo compound Ce2RhIn8 by means of neutron experiments. U2RhIn8 and URhIn5 behave like transition-metal antiferromagnets, contrary to Ce(4f)-counterparts. The 5f-states are entirely hybridized and Kondo interactions play no role. Neither does it in the investigated RE2CoIn8 (RE = Pr, Nd, Dy) compounds which exhibit strong localized 4f-electron behavior. Here, the influence of strong...